Bond Energy



Ozone

In the photolysis of ozone, one of the oxygen-oxygen bonds in the molecule breaks. A specific quantity of energy must be added to break the bond. This is the bond energy.



We can determine the energy necessary to break the O-O bond in ozone with an experiment.

Remember that the energy in light is proportional to its frequency and inversely proportional to the wavelength the light.
    E = hc/
Lasers can emit light of a single wavelength, and certain types of lasers can be tuned so that they emit light over a range of wavelengths. By changing the wavelength of laser light through a sample until the radiation causes the O-O bond to break, we can experimentally determine the exact bond energy.

Absorption spectroscopy will tell us when absorption and bond-breaking happens.

When we find the longest wavelength (lowest energy) radiation that can break the bond, we can use it to calculate the bond energy.


Tunable laser

For 1 molecule:
    E = hc/
    c = (3.0 x 108 m/s)(1.0 x 109 nm/m) = 3.0 x 1017 nm/s
    h = 6.63 x 10-34 J s = 6.63 x 10-37 kJ s
For 1 mole, multiply the energy required to break the bond in 1 molecule by the number of molecules in a mole:
    E = (hc/ molecule)(6.02 x 1023 molecules/mol)
    E = (6.63 x 10-37 kJ s)(3.0 x 1017 nm/s)(6.02 x 1023 mol-1)/
    E = 1.2 x 105 kJ nm mol-1/

The lowest energy light that can break the bond in ozone has a wavelength of 330 nm.
    BE (ozone) = 1.2 x 105 kJ nm mol-1/330 nm = 364 kJ/mol



Other O-O Bonds

We can obtain experimental data on bond energies of other molecules in the same way. Molecular oxygen, O2, is photolyzed by light of 241 nm and has a bond energy of 498 kJ/mol. Hydrogen peroxide, HOOH, has a very weak O-O bond and is photolyzed by light of 845 nm. Its bond energy is only about 142 kJ/mol.

Why do we see such large difference in the strength of oxygen-oxygen bonds in these molecules. Let's look at the Lewis structures.



The bond energy correlates with the bond order.

Bond energies of some gas phase molecules are listed below.

O2 498 kJ/mol
O3 364 kJ/mol
HO-OH 142 kJ/mol
HO-H 459 kJ/mol
H3C-H 435 kJ/mol
ON-O 300 kJ/mol
O2N-O 190 kJ/mol
Cl-Cl 253 kJ/mol
H2N-NH2 160 kJ/mol
N2 941 kJ/mol



Professor Patricia Shapley, University of Illinois, 2012